1. Field of the Invention
The present invention relates to a power steering apparatus which applies a steering assist force to a steering mechanism by a hydraulic pressure generated by a pump driven by an electric motor.
2. Description of Related Art
Power steering apparatuses are conventionally utilized which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor such as a brushless motor, and a steering assist force is generated by the power cylinder in accordance with the rotation speed of the electric motor.
The electric motor is supplied with a driving current via a driving circuit, which is operative on the basis of a driving pulse signal applied from a controller such as an electronic control unit (ECU). The controller generates a driving pulse signal of a high duty ratio when a load on the motor is high, and generates a driving pulse signal of a low duty ratio when the load is small. Thus, a pulse width modulation (PWM) control is conducted to drive the electric motor at a constant rotation speed irrespective of the magnitude of the load. In this case, the detection of the magnitude of the load is achieved, for example, by detecting a motor current.
When a target rotation speed of the motor is low and the load is high, the PWM control is conducted under a high electric current condition. However, if this state continues for a long time, electronic components in the controller suffer from a remarkable temperature rise thereby being damaged. This is because the PWM control under the high electric current condition maximizes the switching loss of a switching device, a steady-state loss, and a loss due to a flywheel current.
In order to eliminate the switching loss and the loss due to the flywheel current under the high current condition, it is a conventional practice to set the PWM duty to 100% and discontinue the pulse width modulation control when the load on the electric motor exceeds a predetermined level. This approach is advantageous in that the electronic components in the controller can be protected and less expensive electronic parts can be employed as the components of the controller.
However, if the load gradually increases to exceed a certain level, the PWM duty is abruptly increased to 100%, whereby the steering assist force to be applied to the steering mechanism is suddenly increased. This gives a driver such an impression that the steering is suddenly lightened, resulting in an unsatisfactory steering feeling.